If your typical workout routine never changes, you're missing out. Here's what you should be doing at the gym, based on your goals and your age

"Honor the fact that your body is going through a normal, natural process, and pay more attention to knowing your physical self well," says Brizendine. Fluctuating hormone levels can lead to mood swings, and exercise is an excellent remedy. "Weight training causes the body to release substance 'x' -- medicine doesn't really know what it is," she explains. "But it improves mood."

I'm in my 20's and I think it's really important to vary your routine and only do things that you enjoy, then you're more likely to stick with it. Also changing up your routine confuses your body and you are less likely to get used to doing the same exercise over and over, then you hit a plateau and have to change your routine. Some days I do Zumba, walk my dog, cardio kick-boxing or a more intense strength training workout. Even if I'm super lazy I'll still fit in a short walk or yoga, once you get going you almost always want to continue. It's good to have a workout buddy, if you know someone is depending on you to show up, you're more likely to follow through. Also check your local parks and recreation. Often they have free classes. Mine offers free yoga and Zumba. They also have great hiking trails which are free, so no need to pay for a gym membership.